(1) Field of the Invention
The invention relates to processes for the manufacture of semiconductor devices and more particularly to processes for forming contacts and Vias.
(2) Description of Prior Art
The integrated circuit (IC) manufacturing industry continues relentlessly towards smaller device geometries and greater circuit densities. This trend is made possible by the development of new manufacturing techniques as well as innovative improvements of existing procedures thereby extending their utility further towards miniaturization and higher density. The benefits and rewards of these efforts in very large scale integrated circuit technology development are extraordinary. Not only are the integrated circuits of today cheaper to produce, they continue to reward both end user and manufacturer with improved reliability and performance.
One such discipline wherein the limits of technology are constantly tested is the formation of openings in insulative layers wherein contacts to subjacent semiconductive elements are made. These openings generally represent the smallest photolithographically defined features of the integrated circuit. The openings are typically formed by reactive ion etching (RIE) through the insulative layer using a patterned photoresist mask. RIE is a well known anisotropic etching technique which can provide deep vertical openings having high aspect ratios. The aspect ratio in this regard is defined as the depth of the opening divided by its width.
The evolution in the capabilities of advanced optical lithography systems using laser light sources with wavelengths in the deep ultraviolet (DUV) spectrum has enabled the resolution of features below 0.25 microns. In spite of the improvements in lens quality, however, these resolutions come at the expense of very shallow depth of focus.
The formation of contact openings for sub-quarter micron IC technology requires such a fine photolithographic resolution. The narrow depth of focus dictates the use of photoresist layers having a thickness no greater than, and preferably less than the depth of focus. In order to achieve a resolution of less that 0.25 microns it is therefore necessary that the photoresist thickness be less than about 5,000 Angstroms. The etch rate selectivities of typically used insulative materials, such as silicon oxide and borophosphosilicate glass (BPSG), with respect to the photoresist materials used in deep-ultraviolet (DUV) photolithography are not sufficiently high to permit such thin photoresist layers to be used alone to accomplish the RIE patterning of contact or via openings.
Instead, a more durable material must be deposited over the insulative layer. This material is then patterned by a thin photoresist mask to form a hardmask. The hardmask material, having a substantially lower etch rate in the fluorocarbon gases used to etch the insulative layer, may be deposited relatively thinly and can therefore be easily patterned with a thin photoresist mask. One of the current inventors, has disclosed titanium nitride(TiN) as a material which forms an effective hardmask for etching of insulative layers and can be conveniently patterned with thin photoresist layers. Whereas the selectivity of silicon oxide to photoresist is only about 2-3:1, the selectivity of silicon oxide to TiN is about 10:1.
A problem with the use of sputtered TiN as a hardmask for anisotropic etching of ILD and IMD layers is that the sputter deposited TiN films lack the carbon content necessary for the formation of a protective polymer on the sidewalls of the contact/via openings during the plasma etch. As a result, the sidewalls are not vertical, suffer from unacceptable surface roughness and image distortion.
FIG. 1 is a cross section of a wafer 10 of with a contact opening 16 etched in an insulative layer 12 with a conventional fluorocarbon etchant containing CF.sub.4 and CHF.sub.3 using a sputtered TiN hardmask 14 which has been patterned by DUV photolithography. The sputtered TiN hardmask does not contain carbon and therefore a sidewall polymer is not formed during the insulator etching. The absence of sidewall protection permits lateral chemical etching resulting in rough and irregular wall surfaces as well as loss of image dimensional integrity.
The use of hardmasks for patterning layers by plasma etching is not new. Dennison, U.S. Pat. No. 5,362,666 shows a method of producing a self-aligned contact penetrating a cell plate using a silicon nitride or polysilicon hardmask to etch a contact hole. Chang, U.S. Pat. No. 5,612,240 shows a contact method using a silicon nitride spacer as a hardmask.
MOCVD(metal organic chemical vapor deposition) methods for forming conductive layers have been reported. For example Chao, et.al, U.S. Pat. No. 5,385,868 cites the formation of aluminum/silicon and aluminum/copper alloy layers by MOCVD. Sandhu, U.S. Pat No. 5,254,499 cites the formation of conformal high density TiN barrier layer films by MOCVD. Shapiro, et.al. U.S. Pat. No. 5,603,988 likewise cites the formation of TaN and TiN films by MOCVD. Generally, the carbon content of these films is problematic because it causes the films to be more resistive than sputtered TaN and TiN films. The predominant use of these films is for metal diffusion barriers in contacts. In this application a high conductivity is sought. Sandhu, U.S. Pat. No. 5,480,684 cites a method for reducing the carbon content of MOCVD TiN films from 21 atomic percent to 12 atomic percent by ion implantation of ions of a "late transition metal", for example platinum, into the MOCVD TiN followed by annealing in hydrogen.
C. Y. Chang and S. M. Sze, in ULSI Technology, McGraw-Hill, New York, (1996) p.388-389 cite the formation of MOCVD TiN films from TDMAT and TDEAT precursors, indicating that these films have low densities and high resistivities because of carbon and oxygen inclusion, compared with sputtered TiN films making them unsuitable as barrier layers.
In the current invention, wherein MOCVD TiN films are used as hardmasks, the incorporated carbon is used advantageously to form a protective sidewall polymer coating during RIE of insulative layers. Conductivity and high conformality are of little or no consequence to a RIE hardmask.